1. Field of the Invention
This invention relates to a disc brake, and more particularly to an anti-squeal shim structure therein for eliminating "brake squeal" and for heat insulation.
2. Prior art
A disc brake generally includes a disc rotor rotating with a wheel of an automotive vehicle, a pair of friction pads each having a backing plate and a friction member secured to the disc rotor side of the backing plate, a caliper secured to a support member of the vehicle and supporting the friction pads movably toward and away from opposite friction surfaces of the disc rotor, and hydraulic actuating means urging the friction pads through forcing members such as a brake piston and a finger of the caliper against the friction surfaces of the disc rotor to restrain the rotation of the disc rotor.
With such a construction, a disc brake may under some conditions emit squealing noise during operation. This brake squeal noise is considered to be caused by relative displacements between the backing plates of the friction pads and the forcing members, and by vibrations due to an interaction between the friction pads and the disc rotor when the friction pads are pressed against the disc rotor.
To reduce or eliminate such brake squeal noise, anti-squeal shims are generally each made of a steel plate with damping material layers applied to both faces thereof, where the anti-squeal shims have been interposed between the friction pads and the forcing members. In most cases, each damping material layer of the anti-squeal shims is formed only of a rubber and selected to be about 100 microns thick. One of the present inventors have disclosed another type of anti-squeal shim, in Japanese Patent Laying-open Publication No. Showa 63-101530, wherein the damping material layers are formed of a compound including a nonmetal fiber other than asbestos, an elastomer and a filler. It should be noted that, in either case, the prior art anti-squeal shims have both faces thereof coated with the damping material layers.
On the other hand, generally, the contact area between the anti-squeal shim and the forcing member such as a brake piston or a finger of the caliper is relatively small. For example, the brake piston is usually hollow with a thin wall shell with the open end thereof opposed to the anti-squeal shim, and thereby the contact area between the anti-squeal shim and the piston forms a ring shape. Accordingly, in the case of a conventional anti-squeal shim coated on both faces thereof with the damping material layers, an extremely high pressure will be partially applied to the damping material layer on the side of the forcing member, if the anti-squeal shim and the forcing member are directly opposed to each other. To obviate the above-mentioned disadvantage, another shim, so-called "backing-up shim", has heretofore been interposed between the anti-squeal shim and the forcing member.
However, the disc brake apparatus equipped with the backing-up shim has the following disadvantages:
(a) The numbers of parts as well as manufacturing costs are increased. PA1 (b) Even if the anti-squeal shim is provided with holes therethrough or recesses at the periphery thereof to improve the heat insulating effect, the desired effect can not be produced because the backing-up shim covers the holes or recesses.